It is known from laboratory studies and field trials that elemental sulfur reacts aggressively with metals in the presence of water or aqueous solution. The accelerated attack on metals caused by elemental sulfur results in pitting, stress cracking, and mass-loss corrosion. According to the Federal Highway Administration study entitled Corrosion Costs and Preventive Strategies in the United States, the total annual estimated direct cost of corrosion in the U.S. in 1998 was approximately $276 billion (approximately 3.1% of U.S. gross domestic product).
Elemental sulfur is a strong oxidizer, causing corrosion where it attaches to the wet steel surface. Elemental sulfur can occur when oxygen mixes with H2S or may be produced naturally. There are few, if any, commercial inhibitors that effectively protect against corrosion caused by elemental sulfur.
The trend to use more ImAg (immersion silver) surface finish and shy away from SnPb HASL (Stannum Lead AntiHot Air Solder Leveling) on electronic products has resulted in corrosion failure occurrences when these products are exposed to high sulfur environments under elevated humidity. The resulting creep corrosion constituent is primarily Cu2S which is produced by galvanic driven attack of the copper beneath the edge of the soldermask. Electronic hardware manufacturers are experiencing, or will soon experience product reliability problems due to sulfur corrosion at tire burning factories, paper mills, fertilizer plants, and polluted locations in developing countries. This new unexpected failure mechanism demands a controlled process by which products can be qualified to ensure they will not fail in these applications.
Prior methods to combat corrosion have included protective organic coatings, cement, sacrificial anodes, various inhibitors cathodic protection, and spray-coating corrosion-susceptible surfaces with corrosion-resistant metals. These methods have been variably effective at reducing corrosion rates and come with variable costs and safety considerations. For example, imidazoline-based inhibitors have been shown to be ineffective in controlling the accelerated localized attack caused by elemental sulfur while chromate and hydrazine are effective for inhibiting corrosion but are carcinogenic.
The inhibition of corrosion on corrodible surfaces in contact with sulfur-containing materials, in solid, semi-solid, liquid, or vapor form has proven to be extremely challenging. The difficulty can in part be attributed to the fact that these materials contain elemental sulfur, sulfur compounds, and other corrosive elements such as salts, acids, and corrosive gases which contact corrodible surfaces that give up electrons, becoming themselves positively charged ions in an electrochemical reaction. When concentrated locally, this reaction forms a pit or a crack, but may also extend across a wide area to produce general corrosion.
Chemical and biological origins are considered to be involved in the corrosion-causing sulfur reactions. The production of sulfur from the chemical origin is governed by the oxidation-reduction potential and pH, while bacteria, such as sulfur bacteria, participate in the formation of sulfur from biological origin.
It is widely recognized that microorganisms attach to, form films on, and influence corrosion on surfaces, especially in aqueous environments. Microorganisms causing sulfur-based corrosion include Halothiobacillus neapolitanus, Thiobacillus ferroxidans Acidothiobacillus thiooxidans, Ferrobacillus ferrooxidans, Thiobacillus thiooxidans, Thiobacillus thioparus, Thiobacillus concretivorus, Desulfovibrio and Desulfotomaculum, Sphaerotilus, Gallionella, Leptothrix, Crenothrix, Clonothrix, and Siderocapsa. The microorganisms change the electrochemical conditions at the surface which may induce localized corrosion and change the rate of general corrosion. Some microorganisms reduce sulfate and produce hydrogen sulfide or oxidize H2S gas to solid sulfur, which can lead to corrosion. Some bacteria produce acids and other corrosive compounds on the surfaces leading to even further corrosion. In addition to metal surfaces, microbial corrosion can also apply to plastics, concrete, and many other materials. The use of surfactants to suppress sulfur bacteria have proven ineffective for longer periods of time, requiring service within a year of use (Kudo and Yuno, Proceedings World Geothermal Congress, 2000).
Formation of scale caused by sulfur or sulfur compounds or from corrosion products adhering to the inner surfaces of pipes serves to decrease ability to transfer heat and to increase the pressure drop for flowing fluids. Also, in the presence of other impurities such as ions of calcium and magnesium in a liquid, e.g., water, gives rise to the formation of scale or voluminous precipitate, thus fouling the surfaces. Scale is an assemblage of precipitates that adhere to surfaces along water paths. Accumulated solid layers of impermeable scale can line pipes and tubes, sometimes completely blocking flow. Metal sulfates, e.g., barium and calcium sulfates, form the most persistent scale, which often requires shutdown of operations for mechanical removal from the metal walls of pipes, boilers, refinery equipment, production tubing, tanks, valves, etc. In boilers, scale results in reduced heat transmission, higher fuel usage, pipe blockage, and local overheating which can damage the boilers. In industrial operations, scale buildup reduces output, puts pressure on pumps, turbines and propellers, and engines, and eventually requires systemic shutdown for scale removal. Thus, in addition to direct removal costs, the indirect costs of scaling are enormous in terms of equipment damage, reduced efficiency, and deterred production. Thus, it is preferable to prevent or reduce scaling as much as possible.
Chemical treatment is often the first approach in attempts to inhibit, reduce, or remove scale. It is more advantageous when conventional mechanical methods are ineffective or expensive to deploy. Prior chemical techniques include contacting the substrate with alkaline salts, acids, inhibitors such as phosphate compounds, chelating solutions, and dispersing agents. Such methods tend to often be ineffective and sometimes dangerous or impractical. Often, not enough scale is removed or inhibited or the chemicals are not compatible with the systems requiring treatment. Hydrochloric acid is often the first choice for scale treatment, but the acid reaction produces by-products which are excellent initiators for reformation of scale deposits. Further, in acid descaling, the system must be shut down, drained, acid cleaned, rinsed, drained and retreated. Ethylenediamenetetraacetic acid (EDTA), a chelator, is also commonly used to stoichiometrically sequester metal ions; however, EDTA is slower than hydrochloric acid and stoichiometric treatments require significant concentrations to prevent scale formation. Because of the disadvantages of chemical treatments, they have sometimes been abandoned in favor of, or combined with, mechanical techniques to remove or reduce scaling.
Earlier mechanical techniques for scale removal included explosives to rattle and break off scale, but often damaged the substrate and did not effectively remove scale. Modern mechanical techniques include shot blasting, abrasive blasting, water jetting, pressurized air blasting, grinding, milling, impact hammering, and shock waves. These tools require full access to the substrate surfaces plagued by scaling and are seldom effective at completely removing scale to the bare walls. Residual scale on surfaces encourages new growth and makes scale inhibitor treatments more difficult. Further, such methods, especially abrasives, can damage substrate surfaces.
Prior techniques have failed to safely and effectively prevent, reduce, or remove scale and corrosion. A need exists for methods and compositions to inhibit corrosion of corrodible surfaces by preventing deposition and/or removing sulfur and other corrosive molecules from corrodible surfaces. There is also a need for scale-prevention, scale-reduction, and scale-removal techniques that are more effective, quick, and non-damaging to the substrate and environment.